1. Field of the Invention
This invention relates generally to satellite constellation designs and more particularly to repeating common ground track constellations which are capable of being sun-synchronous.
2. Description of Related Art
Most of today's satellite constellations have an earth-centric inertial (celestial or geocentric inertial [GCI]) design. In other words, GCI constellations designs are organized into planes having multiple satellites that are fixed in inertial space. The GCI coordinate system is the most common coordinate system for describing a single satellite orbiting the earth.
A popular example of an earth-centric inertial designed constellation is the “Walker” constellation, named after John Walker a pioneer in satellite constellation design. Walker constellations consists of circular orbits of symmetric groups of satellites, uniquely identified by five parameters: (1) altitude, (2) inclination, and the integer triplet T/P/F, where (3) T is the total number of satellites in the constellations, (4) P is the number of planes, and (5) F is the phasing between adjacent planes.
The problem with most earth-centric, inertial constellations of satellites, and especially the Walker constellations, is that they consist of optimal configurations for high-precision earth-imagery applications.
High-precision earth-imagery, such as the successive synthetic-aperture radar (SAR) change detection technology, the coherent change detection (CCD) technology, and the high-level Digital Terrain Elevation Data (DTED) mapping all require that the sensors onboard a satellite pass over an area to be imaged along the same path a multiple number of times. For instance, CCD even requires that two images to be perfectly registered at the pixel level.
Any satellite system designed to perform image collection, whether electro-optics (E/O) or radar based, can arguably detect slow moving objects by CCD. For example, during wartime and perhaps even during peacetime reconnaissance missions, high-precision imagery of a specified region, an area of interest, is often desired. However, the frequency of revisits to this particular target region must be maximized within a minimum period of time so as not to miss any pertinent data.
For instance, a conventional technique for detecting slow moving targets, which has been used by intelligence personnel for more than 50 years, is to search for changes in common images. Change detection, as this process might be referred, allows one to define slow moving targets by analyzing successive SAR or DTED or optical images. Also, areas where tunnels or underground bunkers are under construction have been located by comparing temporally displaced optical images.
However, most earth-centric inertial constellations fail to simultaneously exhibit both high revisit rates and movement along the same ground path. On the other hand, the most notable exceptions to this rule are the constellations known as geostationary satellite constellations, geosynchronous satellite constellations and the equatorial orbiting satellite constellations.
However, these exceptions either require that the satellites orbit at extremely high altitudes (near 35,786 km), the geostationary and geosynchrounous satellites for example, or limits coverage to a very narrow region near the equator, the equatorial orbiting satellites (inclination=0 degrees) for example. However, satellites with altitudes below 35,786 km or with inclinations other than 0 degrees have failed to exhibit high revisit rates for specific targets while showing movement along the same ground paths.
Another problem recognized is that satellite systems which have been used for reconnaissance have not been designed with the object of performing change detection imagery with minimal blurring and distortion.
As shown in FIG. 1, conventional satellite systems are generally organized into a single orbital plane, wherein such a plane is uniquely defined by (1) an inclination angle, i, relative to the earth's equator and (2) an angle of the Right Ascension of the Ascending Node, RAAN. Orbital parameters such as these are used to describe a satellite's orbit and a constellation's configuration. For example, the inclination, i, is a constant defining the angle at which the orbital plane intersects the equator. The RAAN defines an angle between a non-rotating celestial reference, i.e., the first point of Aries, and the line of nodes. The line of nodes is defined by a line formed using the intersection of an orbital plane and the plane of the equator. The line of nodes provides an orbit orientation. All satellites with common values for i and RAAN are said to be in the same orbital plane.
However, the problem with groups of satellites organized into the same orbital plane is that the ground tracks, i.e., the movement of beams across the surface of the earth or the path across the earth, are not common. For example, as described above, the Walker satellite constellation fails to have a common ground track for each of the satellites in the constellation.
Furthermore, as shown in FIG. 2, even when the satellites are organized into different planes, a common ground track is seldom achieved. For instance, the Walker constellation, which includes a globally symmetric collection of satellites, and is aimed at providing groups of satellites that are organized into planes having common values for inclination and the RAANs are equally spaced around the earth. However, such a Walker configuration is sub-optimal because of the restriction that the phasing parameter F must be an integer in the range of 0 and P−1, where P is the number of planes being used.
As previously pointed out, Walker orbits are the classical way for describing satellite orbital parameters. As further shown in FIG. 2, Walker constellations consist of a plurality of equally spaced circular orbits having the same orbital inclinations. The inclination angle, i, of all the orbital planes in a Walker constellation is relative to a reference plane, which is typically the equator of the earth.
In FIG. 2, the orbital planes in the Walker constellations all have an equal planar spacing, i.e., 360°/P, where P is equal to the number of orbital planes. In this way, all of the satellites are equally spaced along the respective orbital plane, e.g., orbital plane 1, by 360°P/T, where T is the number of satellites. The phasing difference between the satellites in adjacent planes 1 and 2 of FIG. 2, referenced against the equator, is 360° F./T, where again F is the phasing parameter, which consists of an integer, and T is the total number of satellites. With this Walker arrangement, the ground tracks of the collection of satellites are seldom common.
The above-noted uncommon ground tracks are clearly illustrated in FIG. 3. As shown in FIG. 3, the ground tracks for a 2/1/0 Walker orbit is illustrated. The phrase 2/1/0 represents 2 satellites in one plane with zero phasing. The orbital altitude that was selected in FIG. 3 was 10,349.56 km, because at this altitude the ground tracks of any satellite in the constellation will retrace itself only once in a 24-hour period. In other words, in this example, the same path, i.e., a common ground track, can not be followed by any other satellite in less than a 24-hour period.
Furthermore, observe that there are six distinct ground traces in FIG. 3. Three ground traces carved out by satellite 1 and three ground traces carved out by satellite 2. The selection of the altitude of 10,349 Km causes the satellite ground traces to close upon themselves after three revolutions around the earth. Ground traces close upon themselves when the ground tracks start to literally follow on top of the previous ground trace. The closure of the ground traces upon themselves in a finite number of orbits is not a requirement for constellation design, but was merely chosen here to simplify the analysis of ground traces.
In FIG. 3, the ground track for satellite 1 has three distinctive ground traces before the ground tracks start to repeat. Likewise, satellite 2 has three other distinctive ground traces before the ground tracks start to repeat. The selection of the altitude helps to determine when a ground track will close upon itself. Satellite 1 and satellite 2 fail to have a common ground track/path across the earth. At the 10,349.56 km altitude example, each ground track consists of 3 ground traces covering 2π radians (360°) of longitude.
When images from two different satellites are taken of a common region of the earth, combining these images to arrive at a detection image will produce blurring and distortion if the two satellites follow different ground tracks. One technique which has been used to remove such defects in the detection images is the morphing technique. For example, “morphing” is used to stretch and twist the images to compensate for the defects. However, even when morphing is performed, there still remain errors in the change detection image due to the fact that the images are of three-dimensional regions, not two-dimensional regions. The stretching and twisting of images by using the morphing technique will compensate for blurring and distortion of two-dimensional images but does not adequately compensate for three-dimensional images.
Yet still another problem associated with conventional satellite constellation designs is that satellites orbiting at low earth orbits (LEO) are typically eclipsed from the sun at least once per orbit. In other words, in most low earth orbits (LEO), the satellites are eclipsed from the sun on the order of 40 minutes out of the nominal 90-minute orbital period. This creates a problem because the radar on most satellites must operate independently of whether or not it is in the sunlight. This means that any solar array on the satellite must be augmented by energy storage batteries in order to maintain power.
Most LEO satellite systems are designed to operate only a small fraction of the orbit so that the total weight of the power system is minimized. Under this scenario, the solar array is designed to trickle charge the batteries throughout the time it is in the sunlight and the batteries are sized to provide the power to the radar. Since batteries are very heavy in comparison to the solar array, the optimal solution under this scenario is that a small fraction of the orbital period is used for radar operation while leaving a larger fraction of the orbital period for charging the batteries. For long operational time period requirements, the satellite power system favors drawing the power from the solar arrays versus from the batteries because, for the same energy storage requirements, batteries weigh far more than the solar arrays. This means, for typical satellite orbits the amount of time that one operates while eclipsed would have to be sacrificed. It is desirable for the radar in the satellites to operate all of the time, but weight concerns tend to limit the ability to achieve this goal.
What is needed is a satellite constellation that provides high-precision imagery and high-level DTED mapping of specified regions on the earth at high revisit rates within a specified period while being capable of minimizing satellite battery requirements.